The invention herein relates to refractory fiber compositions. More particularly, it relates to a refractory fiber composition suitable for use in an intermediate range fibrous thermal insulation.
Alumino-silicate fibers containing essentially no components except alumina and silica have been widely used for some years for thermal insulation capable of withstanding temperatures up to approximately 2300.degree. F (1260.degree. ); such fibers are described in U.S. Pat. No. 3,456,914. Similarly, alumino-silicate fibers containing certain added oxides, such as chromia, have been used for thermal insulation for temperatures up to approximately 2600.degree. F (1430.degree. C); such fibers are described in U.S. Pat. No. 3,449,137. Such compositions are difficult to fiberize, since the temperature range in which the melt remains in a fiberizable condition is narrow and solidification is rapid. The alumino-silicate fibers are therefore quite expensive and are used only where the high temperature ranges involved require their use.
At the other end of the temperature scale, in the range of approximately 800.degree. F to 1200.degree. F (425.degree. C to 650.degree. ) there are a wide variety of inexpensive and readily formed fibers. Common glass fibers are ordinarily quite suitable for the temperature range of 800.degree. F to 1000.degree. F (425.degree. C to 540.degree. C). Mineral wools (which include slag wools, rock wools and the like) can normally be used at temperatures up to about 1200.degree. F (640.degree. C). The glass fibers are primarily siliceous materials, while the mineral wool materials have a substantial alumino-silicate content but also very large amounts of lime (commonly 30% to 45%) as well as magnesia (2% to 10%). The common raw materials, such as slag and various types of rock, also contain many impurities which substantially limit their maximum temperature usage. Since the principal raw material in the glass fibers is silica, it is economically feasible to use relatively pure materials for the glass fibers. However, the mineral wools cannot economically be made of pure components, such as silica, alumina, magnesia, and lime, but rather must be made of cheap materials such as slag because of cost.
Although both ends of the temperature spectrum are therefore well covered by choices of insulating fibers, there are no satisfactory and economic materials for temperature ranges between the upper and lower ends of the scale. Thus, those users whose temperature requirements call for fibers which are serviceable in the temperature range of 1400.degree. F to 2000.degree. F (760.degree. C to 1100.degree. C) must either forego the use of fiber and switch to some kind of block insulation, or else must use the expensive alumino-silicate fibers. Not only is the latter choice unduly expensive, but the fibers themselves are being used at considerably less than their optimum operating conditions, for they are really designed for service well above 2000.degree. F (1100.degree. C).
The service temperature of fiber is determined by three parameters. The first is the obvious condition that the fiber must not melt or sinter at the temperature specified. It is this criterion which precludes the use of many of the glass and mineral wool fibers at temperatures above 1200.degree. F (650.degree. C). Second, a felt or blanket made from the fibers must not have excessive shrinkage at its service temperature. "Excessive shrinkage" is usually defined to be a maximum of 3% lineal shrinkage after prolonged exposure (usually for 24 hours) at the service temperature. Shrinkage of mats or blankets used as furnace liners and the like is of course a critical feature, for when the mats or blankets shrink they open fissures between them through which the heat can flow, thus defeating the purpose of the insulation. Finally, the third factor governing maximum temperature use is amount of devitrification (partial or complete crystalization) that occurs. The more devitrified a fiber is, the more brittle it becomes, thus eliminating one of the principal advantages of fibers and fiber mats: their flexibility and resilience. Thus, a fiber rated as a "1600.degree. F (870.degree. ) fiber" would be defined as one which does not melt or sinter and which has acceptable shrinkage and degree of devitrification at that temperature, but which begins to suffer in one or more of the standard parameters at temperatures above 1600.degree. F (870.degree. C).
It is thus evident that there is a significant need for a class of refractory fibrous materials which will have service temperatures in the range of 1400.degree. F to 2000.degree. F (760.degree. C to 1100.degree. C). Such fibers would find wide spread use in bulk, mat or blanket form as insulations for furnaces, kilns and the like operated in such temperature ranges. They would also be particularly useful as insulation for the catalytic mufflers used on many automobiles.
U.S. Pat. No. 2,046,764 describes a predominately aluminous self-bonded abrasive containing alumina, silica, lime and magnesia. This material, which is to be used for such things as grinding wheels and floor tread coatings, consists of large crystals of alumina dispersed through a relatively glassy matrix of the other oxides. U.S. Pat. No. 3,402,055 describes glass fibers useful as reinforcing or insulating fibers which comprises a major portion of silica with minor amounts of alumina and magnesia. U.S. Pat. No. 3,819,387 describes hard glass beads intended for blasting surfaces for cleaning and abrading, as well as for making extra hard plates and the like. This composition contains a predominate portion of silica with small amounts of alumina, lime and magnesia as well as other oxides. The silica-to-alumina ratio is at least 3:1. Dolomite may be used to provide lime and magnesia content. Other U.S. patents which describe refractory compositions containing greater or lesser amounts of silica, alumina, magnesia and/or lime in various combinations include U.S. Pat. Nos. 1,966,406; 1,966,407; 1,966,408; 1,818,506 and 2,599,184.